The present invention is directed to the fabrication of semiconductor devices and, more particularly, to the metrology of features formed during the fabrication of such semiconductor devices.
The manufacture of a semiconductor device typically requires a large number of process steps. Each process step includes one or more processing parameters that must be controlled within a relatively narrow range to obtain devices which have the desired characteristics and to obtain an acceptable yield of devices having such characteristics. In addition to controlling the conditions of each process step, the dimensions and structure of various features are determined throughout the fabrication process to ensure that the process conditions remain under control as well as to verify the calibration of various processing systems or tools. Such measurements may be carried out on control wafers, namely non-device wafers that are processed with the device wafers, or on the actual device wafers. Among the features or critical dimensions (CD) are line width, line height, sidewall angle and profile, and trench depth, as well as the presence of open or partially-opened contact windows or vias.
Advances in semiconductor processing materials and techniques have reduced the overall size of the device circuit elements or features while increasing the number of elements on a single chip. The decreased feature size and increased density have made the use of various metrology techniques more critical while also increasing the difficulty of obtaining accurate and repeatable measurements with these techniques. As an example, optical metrology systems typically use incident, scattered or reflected white or monochromatic light to illuminate the feature or structure where a measurement is taken. Such measurements are best taken, namely have the highest contrast, when the area of the feature to be measured is approximately equal to the background i.e., the remaining area that is illuminated. However, as the features of interest have become smaller, the area of these features may become a much smaller proportion of the total illuminated area and causes increased noise in the measurement signal, thereby reducing the sensitivity of the measurement.
Attempts to reduce the total illuminated area, so that the area of the features of interest is a greater proportion of the total illuminated area, include the use of improved lenses and/or the incorporation of apertures or pinholes. However, as the sizes of the features of interest further decrease, further reductions in the total illuminated area are needed.
It is therefore desirable to carry out such optical measurements in which the total illuminated area is further reduced with respect to the size of the features of interest.